The present invention relates to a process for producing a titanium dioxide containing polyester resin composition. More particularly, the present invention relates to a process for producing a polyester resin composition from which a white film useful as a reflective photographic support can be prepared.
White inorganic pigments typified by titanium dioxide are conventionally dispersed in polyester resins typified by polyethylene terephthalate (PET) by either one of the following methods: adding such pigments per se; or suspending them in an ester-forming polyhydric alcohol such as ethylene glycol to prepare a slurry and adding the slurry during the process of esterification or polycondensation. The latter method is disclosed in many prior patents such as Japanese Patent Publication Nos. 945/1958 and 18135/1981. But this method has the problem that the particles of titanium dioxide added tend to agglomerate and precipitate in ethylene glycol or the resulting polymer, and the amount of titanium dioxide that can be incorporated is no more than several percent.
If one wants to incorporate a fairly large amount (.gtoreq.10%) of titanium dioxide in the polyester resin, it is common practice to perform kneading with such equipment as a continuous kneading extruder. Methods for adding titanium dioxide and other white inorganic pigments to polyester resins by means of a continuous kneading extruder are described in Japanese Patent Application (OPI) Nos. 246236/1986 and 250034/1986 (the term "OPI"as used herein means an unexamined published Japanese Patent Application). The method described in Japanese Patent Application (OPI) No. 246236/1986 comprises dry blending a pigment with a polyester resin having a bulk density of no more than 0.6, blending them in molten state to prepare a master batch, and mixing the master batch with another feed of the polyester resin in molten state. However, providing a polyester resin with a bulk density of no more than 0.6 and dry blending it with a pigment are quite cumbersome steps and it frequently occurs that the added pigment particles are not uniformly dispersed in the resin.
The method disclosed in Japanese Patent Application (OPI) No. 250034/1986 comprises mixing a polyester resin with a pigment in molten state to prepare a master batch, subjecting the master batch to solid-phase polymerization, and mixing the resulting polymer with another feed of the polyester resin. This method has the following disadvantages: polymerization requires as long as 5 to 20 hours to complete; the method involves many steps; and as in the first method, it sometimes occurs that the added pigment particles are not uniformly dispersed in the polyester resins. Furthermore, the two methods have a common problem in that the resin can pick up color as it is subjected to two cycles of blending in molten state.